ninth day of the cold season
Refers to the ninth day from the start of the traditional 寒の入り (kan no iri), which falls around January 5th–6th. This day is approximately January 13th–14th and is considered the coldest period. Used in seasonal contexts and traditional calendars.
The time around the ninth day of the cold season is said to be the coldest of the year.
Compound of 寒 (kan, 'cold') and 九 (ku, 'nine'), referring to the ninth day of the traditional cold season.